Conventional microelectronic device packages typically include a microelectronic substrate or die attached to a support member, such as a printed circuit board. Bond pads or other terminals on the die are electrically connected to corresponding terminals of the support member, for example, with wire bonds. The die, the support member, and the wire bonds are then encapsulated with a protective epoxy material to form a device package. The package can then be electrically connected to other microelectronic devices or circuits, for example, in a consumer or industrial electronic product such as a computer.
In one existing arrangement shown in FIG. 1A, a microelectronic device package 10a includes a support member 20 having an aperture 21. A microelectronic substrate 30 is attached to the support member 20 with strips of adhesive tape 40a. Substrate bond pads 31 are then electrically connected to corresponding support member bond pads 22 with wire bonds 32 that extend through the aperture 21. An encapsulant 11, which includes a suspension of filler material particles 12, is disposed over the microelectronic substrate 30 and the wire bonds 32. The sizes of the filler material particles 12 in any given package 10a typically range in a standard distribution about a selected mean value.
One drawback with the foregoing arrangement is that the filler material particles 12 (and in particular, the largest filler material particles 12) can impinge on and damage the microelectronic substrate 30. Because the larger particles 12 tend to settle toward the support members 20, one approach to addressing the foregoing drawback is to increase the separation distance between the microelectronic substrate 30 and the support member 20 by increasing the thickness of the tape 40a. Accordingly, an advantage of the tape 40a is that it can be selected to have a thickness sufficient to provide the desired separation between the microelectronic substrate 30 and the support member 20. However, a drawback with the tape 40a is that it can be expensive to install. A further drawback is that the tape 40a can be difficult to accurately position between the support member 20 and the microelectronic substrate 30.
FIG. 1B illustrates another existing microelectronic device package 10b having a microelectronic substrate 30 attached to the support member 20 with screen printed strips of epoxy 40b. The epoxy 40b can be easier than the tape 40a (FIG. 1A) to dispense on the support member 20, but can have other problems. For example, the epoxy 40b can apply stresses to the sides of the microelectronic substrate 30, but it may be difficult to control how much of the sides the epoxy 40b contacts, making it difficult to control the stress applied to the microelectronic substrate 30. Another drawback is that the thickness of the epoxy 40b typically ranges from about 8 microns to about 25 microns, while in some cases the desired separation between microelectronic substrate 30 and the support member 20 is greater than about 75 microns, for example, to avoid the particle impingement problem described above. Still another drawback is that the interfaces between the epoxy 40b and the encapsulant 11 (one located to the outside of the microelectronic substrate 30 and the other located beneath the microelectronic substrate 30) can delaminate, which can reduce the integrity of the package 10b. The interface located beneath the microelectronic substrate 30 can also create a high stress region that can cause a crack C to form in the encapsulant 11. The crack C can damage the integrity of the wire bond 32.
Another problem with both the tape 40a and the epoxy 40b is that the coefficient of thermal expansion (CTE) of these components is typically substantially different than the CTE of other components of the package. For example, the microelectronic substrate 30 typically has a CTE of about 3 parts per million (ppm) per ° C., the support member 20 typically has a coefficient CTE of about 50 ppm/° C., and the encapsulant 11 typically has a CTE of from about 10-15 ppm/° C. By contrast, the tape 40a and the epoxy 40b each have a CTE of about 150-400 ppm/° C. Accordingly, both the tape 40a and the epoxy 40b can exert substantial shear and/or normal forces on the microelectronic substrate 30 during thermal excursions for curing, reflow and other processes. These forces can crack the microelectronic substrate 30, and/or delaminate layers from the microelectronic substrate 30 and/or the support member 20, causing the package to fail.